Mutations at pipX suppress lethality of PII-deficient mutants of Synechococcus elongatus PCC 7942.

The P(II) proteins are found in all three domains of life as key integrators of signals reflecting the balance of nitrogen and carbon. Genetic inactivation of P(II) proteins is typically associated with severe growth defects or death. However, the molecular basis of these defects depends on the specific functions of the proteins with which P(II) proteins interact to regulate nitrogen metabolism in different organisms. In Synechococcus elongatus PCC 7942, where P(II) forms complexes with the NtcA coactivator PipX, attempts to engineer P(II)-deficient strains failed in a wild-type background but were successful in pipX null mutants. Consistent with the idea that P(II) is essential to counteract the activity of PipX, four different spontaneous mutations in the pipX gene were found in cultures in which glnB had been genetically inactivated.